Method and apparatus for mixing and extruding visco-elastic materials

ABSTRACT

Mixing characteristics of an elastic melt extruder are improved by providing, in the opposing, spaced-apart walls of a rotor and a stator, shaped recesses arranged annularly about the axis of rotation. During processing, the visco-elastic material, generally travelling radially inward between the wall surfaces of the rotor and the stator, is accelerated into the recesses and mixed together before it is caused to move in toward the center of the gap and continue its radial inward movement with the mass of material.

United States Patent Maxwell I Sept. 5, 1972 [54] METHOD AND APPARATUSFOR MIXING AND EXTRUDING VISCO- ELASTIC MATERIALS [72] inventor: BryceMaxwell, Princeton, NJ.

[7 3] Assignee: USM Corporation, Boston, Mass.

[22] Filed: Jan. 22, 1970 21 A l. No.: 4,876

[52] US. Cl ..425/207, 425/376 [51] Int. Cl. ..B29 f 3/012, B29f 1/022[58] Field of Search ....l8/l2 C; 1/30 JM, 376, 30 J8;

[56] References Cited UNITED STATES PATENTS 1 I 3,009,685 11/1961 Rettig..1 8/l2CUX Jurgensen et al. ....-.264/349 X 3/ 1963 Adams ..264/349 X3,253,300 5/1966 Gove et al ..18/12 C X 3,577,841 5/197] Massy ..1s/12cPrimary Examiner-J. Spencer Overholser Assistant Examiner-Norman LehrerAtt0rney--Kenyon & Kenyon Reilly Carr 8; Chapin [57] ABSTRACT Mixingcharacteristics of an elastic melt extruder are improved by providing,in the opposing, spaced-apart vwalls of a rotor and a stator, shapedrecesses arranged 3Claims,6DrawingFigunes PATENTEnsEP 51912 3.689.181

sum 1 or 4 INVENTOR. 456/66 AM 62L PATENTED SEP 5 mg P'A'TE'N'TEDSEF 5m2 SHEET 3 OF 4 A m/V65 METHOD AND APPARATUS FOR MIXING AND EXTRUDINGVISCO-ELASTIC MATERIALS BACKGROUND OF THE INVENTION The presentinvention relates to the extrusion of visco-elastic materials, and moreparticularly to a method and apparatus for accomplishing mixing of suchmaterials prior to final extrusion.

The extrusion of materials includes the mixing, compounding andtransporting of materials, as well as the changing of the conditions orstate of materials by the addition of heat and pressure. A type ofmixing extruder, commonly known as the elastic melt extruder, isdescribed in the Modern Plastics Magazine issue of October 1959, in anarticle by Bryce Maxwell and Anthony J. Scalora, and in US. Pat. No.3,046,603 issued to Bryce Maxwell on July 31, 1962. The elastic meltextruder utilizes the normal force effect based on the principle thatwhen a viscoelastic material is sheared between a rotating surface and arelatively stationary surface, a centripetal force is developed whichcauses the material being processed to be extruded through an outletcentrally located in one of the two members.

More particularly, in the general method of extrusion, plastic pelletsor other materials of a similar nature are fed through a hopper to acup-shaped container, sometimes referred to as a stator. Heatingelements can be employed to assist in melting the pellets into a liquidmass. A cylindrical rotor is rotatably mounted in the container with itsbottom face spaced apart from the bottom wall of the container toprovide a gap through which the liquid flows. Also, a discharge openingis provided in the bottom of the container near the axis of rotation ofthe rotor. Rotation of the rotor relative to the container causes theliquid in this gap to become sheared. As the rotational speed isincreased, the shearing stress is increased, and a force normal to thisstress is developed between the bottom face of the rotor and the bottomwall of the container. The force developed results from a centripetalpumping action which produces sufficient force on the visco-elasticmaterial to effect the extrusion. Also, the rotor is anchored againstaxial displacement so that this normal force will be efiectively used inthe pumping and extrusion of the material out of the discharge openingthrough an extrusion die.

While the mixing characteristics of the conventional elastic meltextruder are generally very good, due to the volume flow of materialbeing perpendicular to the direction of shear, it has been found thatthe material located adjacent and in contact with the surfaces of therotating member and the relatively stationary member, hereinafterreferred to as the rotor and the stator, respectively, are not very wellmixed since the centripetal velocity of such materialsis at a minimum atthe surfaces of such rotor and stator, and since there is little mixingmovement perpendicular to such surfaces.

OBJECTS Therefore, it is an object of the present invention to provide amethod and apparatus for improving the mixing characteristics of theelastic melt extruder.

It is a further object to provide a method and apparatus for improvingthe mixing characteristics of the elastic melt extruder, particularly inthe areas adjacent to the surfaces of the rotor and stator.

SUMMARY OF THE INVENTION These and other objects which will becomeapparent from the detailed disclosure and claims to follow are achievedby the present invention which provides a method for mixingvisco-elastic materials in an elastic melt extruder, comprising feedinga visco-elastic material into the gap formed between the rotor and stavtor surfaces of the extruder and causing an'alternating mixing of thematerial near the rotor and stator wall surfaces as such material iscentripetally accelerated towards the axis of rotation of said rotor.

Also, the present invention provides in an apparatus for extruding avisco-elastic material, first and second 7 members with walls providingsurfaces spaced'from and facing each other in close proximity; means forrotating at least one of the members relative to the other to shear thevisco-elastic material between such surfaces at a rate sufficient toproduce a centripetal force in excess of the centrifugal force on thematerial and thereby cause a flow of said materialtoward the axis ofrotation; means for feeding visco-elastic material into the gap betweensaid surfaces; and a discharge opening formed through one of saidmembers near the axis of rotation; the improvement of which comprises:providing in such surfaces of the first and second members, recessesdefined by recessed surface portions ex-v THE DRAWINGS FIG. 1 is avertical cross-sectional view of the elastic melt extruder, illustrativeof the invention;

FIG. 2 is a vertical view, taken'on line 2-2 of FIG. 1, showing theannular recess areas formed in the'stator wall;

FIG. 3 is a cross-sectional view of the gap formed between the rotor andstator surfaces, with the centripetal velocity profiles indicated atvarious points in the gap;

FIG. 4A shows a close-up view of a centripetal velocity profilev of thematerial in the gap near the beginning of the recess; and FIG. 4B showsthe paths of the particles of material in the area of the recess; and

FIG. 5 shows a cross-sectional view of another embodiment of the rotorand stator design of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings,there is shown, in FIGS. 1 and 2, the elastic melt extruder 10 of theinvention. Extruder 10 comprises two members 12 and 14 providing arelative rotative motion therebetween, such as a rotor 12 and a stator14. Rotor 12 and stator 14 have working surfaces 16 and 18,respectively, spaced apart from each other and facing each other inclose proximity so as to provide a gap 20 between such surfaces 16 and18. Stator 14 is fixedly attached to stationary member 22, while rotor12 rotates on bearings 24 about an axis of rotation 26 as it is drivenby a motor 28. The materi' al for processing is fed into a materialsupply hopper 30 mounted on top of the stationary member 22 with itsopening in communication with a hole 32 extending through the topportion of member 30 and terminating in the gap 20. The material thuspasses through the opening 32 and into the gap 20 at a point near theperiphery of the rotor 12.

It is to be understood that means suitable for driving the rotor 12 andfor feeding material into the gap 20, other than those shown anddescribed, can be employed in carrying out the invention. Also, heatingmeans, not shown, can be employed to assist in melting the material intoa liquid mass.

A plurality of recesses 34 and 36, and 38 and 40, respectively, areprovided in the surfaces 16 and 18, respectively. These recesses 34, 36,38 and 40 are formed by surfaces of revolution about the axis ofrotation 26. Preferably, the recesses 34 and 36 on the rotor 12 arealternated between the recesses 38 and 40 along the gap 20 ina-generallyradial direction. Thus, the diameter of the annularringformed by each recess varies in size from the rotor to the stator andback to the rotor, and so on, as the ring diameters increase ordecrease. In this fashion, as the material. passes from the hopper 30towards a discharge opening 42 provided in the stator 14 near thelocation of the axis of rotation 26, such material'will first pass bythe recess 34 on the rotor 12, then the recess 38 on stator 14, then therecess 36 on rotor 12 and finally the recess 40 on stator 14, thusalternating from side to side across the gap 20.

Referring to FIG. 3, there is shown the velocity profiles of thematerial at various locations in the gap 20. Specifically, the radialvelocity profiles 44, 46, 48, 50 and52 indicate the relative magnitudesof the centripetal velocity of the material at points across the gapbetween the rotor 12 and stator 14 where the recess are not provided. Asindicated by the velocity profiles 44-52, the centripetal velocity ofthe material in the elastic melt extruder is at a minimum at points onthe surfaces 16 and 18. As a result, the material located at the.surfaces 16 and 18 is not effectively mixed into the mass of thevisco-elastic material being processed.

The recesses 34, 36, 38 and 40 provide a means for improving the mixingcharacteristics of the extruder 10 particularly in the areas close tothe surfaces 16 and 18, as illustrated in FIG. 3 by the velocityprofiles 54, 56, 58 and 60. The material located in intimate contactwith and adjacent the surfaces 16 and 18 loses contact with saidsurfaces 16 and 18 as it moves past the recesses 34, 36, 38 and 40 andis accelerated centripetally by the elastic melt effect. Thus, thecentripetal velocity profiles 54, 56, 58 and 60 of the material aregreatly increased in the area of any one of the recess 34, 36, 38 and40. FIG. 4A is a close-up view of the profile54 of the material near thebeginning of the recess 34. This disruption of the normal velocityprofiles 44, 46, 48, 50 and 52 causes the materials that were close tothe surfaces 16 and 18, and in the area of Y the recesses, to first moveinto the recesses and be mixed with adjacent material in a turbulentmanner, then to continue its radially inward movement due to thecentripetal forcing action of the shear field. The paths taken byparticles of the material in one example are illustrated in FIG. 48.

Thus, the overall effect of the recesses 34-40is to produce analternating mixing of the material near the surfaces of the rotor andthe stator.

As noted previously, the recesses 34, 36, 38 and 40 are formed bysurfaces of revolution about the axis of rotation 26. Each recess, suchas 34 shown in FIG. 3, has a shape defined by the recessed wall portionscomprising upstream portion 62 and downstream portion 64. The upstreamrecess portion 62 curves smoothly inward from the wall surface 16, andthe downstream recess portion 64 gradually slopes from within the recess34 out to where it meets the wall surface 16 at the downstream side ofthe recess 34. While variations in the specific shape of the recessedwall portions 62 and 64 will also provide the improved mixingcharacteristics desired, a rectangular shaped recess having sharp comerson the downstream end portion 64 will not be suitable for this purposesince a buildup of the material might occur in the downstream corners ofsuch recess areas. However, the recess shapes shown in FIGS. 4A and 48with a sharp corner on the upstream end. portion and a smooth downstreamend portion which graduallymeets the wall surface 16 will also besuitable.

It is to be pointed out that while the figures show rotor and statorwall surfaces each having two annular recesstherein, either more or lessthan this number can be employed. However, it is desirable that at leastone recess should be provided on both the rotor and stator wall surfacesso as to produce a back and forth movement of the material toward therotor and stator wall surfaces 16 and 18 on at least one occasion.

Referring to FIG. 5, there is shown a modification of the rotor andstator design shown and discussed in reference to FIGS. l-3. Moreparticularly, the rotor 66 and stator 68 provide wall surfaces 70 and 72which are spaced apart so as to form a gap 74. Wall surfaces 70 and 72are contoured to provide a convoluted path for the visco-elasticmaterial so that such material progresses back and forth along obliqueangles with respect to the axis of rotation 76. In this embodiment, aplurality of recesses 78, 80, 82 and 84 are provided in the rotor andstator surfaces 70 and 72 at the locations 86, 88, and 92 where eachbend occurs in the gap 74, and in the inner wall of each bend. As thematerial passes each corner or bend 86-92, the mixing is improved byacceleration of the material along paths 94, 96, 98 and 100. Theprinciples of operation of this embodiment are essentially the same asthose discussed in reference to the embodiment of FIGS. l-4.

While the specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the inventionprinciples, it is to be understood that the invention may be embodiedotherwise without departing from such principles.

I claim:

1. In an apparatus of the type known generally as an elastic meltextruder for extruding a visco-elastic material having first and secondmembers with surfaces spaced from and facing each other in closeproximity; means for rotating at least one of said members relative tothe other to shear the visco-elastic material between said surfaces at arate sufficient to produce a centripetal force in excess of thecentrifugal force on the material and thereby cause a flow of saidmaterial toward the axis of rotation of the rotating member;

recesses, and such material located in the area of said said recessedwall portions defining each recess comprising an upstream portion and adownstream portion, said upstream recess portion curving inward fromsaid wall surface and said downstream recess portion gradually slopingfrom within said recess out to where it meets said wall at thedownstream side of said recess, whereby the visco-elastic material inintimate contact with or adjacent said walls loses contact with saidwall surfaces as it is accelerated centripetally past said recess isalso caused to move away from said recessed wall portions towards thecenter of the gap and be mixed with the mass of material.

2. Apparatus for extruding a visco-elastic material as recited in claim1, wherein the surfaces of said first and second members are contouredto provide a gap between said surfaces which progresses back and forthalong oblique angles with respect tosaid axis of rotation, therebyproviding a convoluted path for the material.

3. Apparatus for extruding a visco-elastic material as recited in claim2, wherein there is provided a recess at each of a plurality of pointswhere a bend is located in said gap, each recess being in the wall onthe inside of the bend at which it is located.

1. In an apparatus of the type known generally as an elastic meltextruder for extruding a visco-elastic material having first and secondmembers with surfaces spaced from and facing each other in closeproximity; means for rotating at least one of said members relative tothe other to shear the visco-elastic material between said surfaces at arate sufficient to produce a centripetal force in excess of thecentrifugal force on the material and thereby cause a flow of saidmaterial toward the axis of rotation of the rotating member; means forfeeding viscoelastic material into the gap between said surfaces; and adischarge opening formed through one of said members near said axis ofrotation; the improvement of which comprises: recesses in said walls ofsaid first and second members, said recesses defined by recessedportions extending inward from said spaced apart surfaces, said recessedportions forming surfaces of revolution about said axis of rotation,said recessed wall portions defining each recess comprising an upstreamportion and a downstream portion, said upstream recess portion curvinginward from said wall surface and said downstream recess portiongradually sloping from within said recess out to where it meets saidwall at the downstream side of said recess, whereby the visco-elasticmaterial in intimate contact with or adjacent said walls loses contactwith said wall surfaces as it is accelerated centripetally past saidrecesses, and such material located in the area of said recess is alsocaused to move away from said recessed wall portions towards the centerof the gap and be mixed with the mass of material.
 2. Apparatus forextruding a visco-elastic material as recited in claim 1, wherein thesurfaces of said first and second members are contoured to provide a gapbetween said surfaces which progresses back and forth along obliqueangles with respect to said axis of rotation, thereby providing aconvoluted Path for the material.
 3. Apparatus for extruding avisco-elastic material as recited in claim 2, wherein there is provideda recess at each of a plurality of points where a bend is located insaid gap, each recess being in the wall on the inside of the bend atwhich it is located.